How I do it

So, how do I get this fine new effect? Well, you need to do three things:

The first thing is to make sure that your list is valid (X)HTML. This means that a UL contains only LIs.
That's right. It doesn't contain other ULs, it doesn't contain text. <li> tags and that is all. This is very important.
So, how do I nest ULs, I hear you ask? Like this:

See that? Each UL contains only LIs and that's it. Text nodes and
ULs are all contained within LIs. You must do this or aqTree2 will not
work.
If you've got a sublist inside a UL, then all the other
stuff in that UL becomes the list header. (See the "Sublist X.Y"
texts in the above example; that text becomes the link to the
submenu.)

Secondly, make your top-level UL of class aqtree2. The LIs within this top-level UL are the ones that are initially displayed.

Thirdly, include the aqtree2 JavaScript file that does the work. Add the following line to the HEAD of your HTML file:<script type="text/javascript" src="aqtree2.js"></script>

You may also customise the plus, minus and bullet images as
follows (include this code in the HEAD of your page, after
you include the aqtree2.js file):
<script type="text/javascript">
aqtree2_expandMeHTML = '<img src="http://www.kryogenix.org/code/browser/aqtree/plus.gif" />';
aqtree2_collapseMeHTML = '<img
src="http://www.kryogenix.org/code/browser/aqtree/minus.gif" />';
aqtree2_bulletHTML = '<img src="http://www.kryogenix.org/code/browser/aqtree/bullet.gif" />';
</script>
The variables can be set to any HTML you choose (so they could just
be '+', '-' and '·', for example; they don't have to be
images).

And that should be it.

A note on browser compatibility:
I've personally tested this in IE5.5/Win, Mozilla 1.0.0/Linux, and
Konqueror 2.2.2/Linux, and it works fine. Opera (all versions) fails completely, probably due to its lack
of DOM support. K-Meleon 0.6.5 is OK. IE6sp1/WinXP is fine but with a
report of screen flicker. I've also had reports that IE5.1.2/Mac
crashes (erk!) on leaving the page. I'm interested in other browser
success/failure reports; let me know at the contact address below.